This invention relates to the field of blockable gas springs which may be used as positioning devices, e.g. for the head adjustment of a steering column in a motor vehicle. If a gas spring is used as a positioning element, it is desirable on the one hand that the adjusting movement in one direction occurs automatically as a result of the biasing action of a pressurized gas. On the other hand, it is also desirable that in an adjusted condition, the length of the gas spring is unvariable against axial forces in both axial directions.